This invention relates to a receptacle or protective cap for the writing point of a tube writing pen.
Receptacles or protective caps for the writing points of tube writing pens have been proposed and in one such protective cap there is provided a substantially cup-shaped sealing insert made of a resilient material such as polyethylene or polypropylene, having a cylindrical upper end forming an edge which, when the writing point has been firmly screwed or fitted into the cap, makes contact with the front edge of a sleeve of the pen which embraces the writing cone.
It transpires that geometrical stability requires the sealing insert to be made of a material that is only slightly resilient, so that the position of the upper edge of the sealing insert, i.e. its sealing face, will not in practice adapt the position of its surface inside the cap to ensure contact with the front edge of the sleeve. The relatively slight elastic resilience quite generally prevents the creation of a good seal between the upper edge of the sealing element and the front edge of the sleeve, or at least no seal is formed after the sealing face has been only once slightly deformed.
In theory this difficulty could be overcome by making the sealing element of a substantially softer and more resilient material. However, in the conventional type of sealing insert this would cause the sealing element to be axially deformed when the cap is screwed over the writing point of the tube writing pen. As such this deformation should not matter, but in practice it appears that axial deformation is accompanied by a deformation of the annular sealing face of the element so that, notwithstanding its soft resilience, a satisfactory seal is still not created between the sealing face and the front edge of the sleeve.